Laser Airborne Depth Sounder (LADS) systems are used to measure water depth, i.e. the distance between a water surface and the bottom located beneath the surface. This depth measuring capability may be extended to also measure the height of land, and it is understood that the following descriptions that refer to the measurement of water depth can also encompass land height measurement.
The LADS systems contain measuring equipment that is mounted within an aircraft. Typically a fixed wing aircraft is used. During flight, the equipment measures over a vast area the depth at “sounding” positions. These sounding positions are typically in the form of a rectilinear grid pattern that has a 5 m×5 m grid spacing. Typical depth of water that can be measured by the systems is up to 70 m. The position of each sounding is determined with a Global Positioning System (GPS) and recorded with the associated measured depth. This “sounding” data is stored on appropriate media for later evaluation, e.g. digital tape.
The measuring equipment includes a laser that is pointed in a direction downward from the aircraft towards the water. The laser light is pulsed and directed to a sounding position by a primary mirror. Oscillating this mirror about two axes whilst the aircraft is flying creates the grid pattern of sounding on water. The spacing between the soundings also depends on the aircraft altitude, the aircraft forward speed, the frequency, and amplitude of oscillation of the primary mirror and the pulse rate of the laser. Due to design constraints of the LADS system, the primary mirror has a relatively large moment of inertia, which limits the rate at which the primary mirror can be oscillated. This limits the minimum spacing that can be achieved between laser soundings.
In co-pending applications, whose contents are incorporated by reference therein, and entitled “An apparatus and method for oscillating a transmitted laser beam of light within the Field of View (FOV) of a light receiving system” and “An apparatus and method for the measurement of water depth using a controlled receiver” the applicant disclosed an improvement in the scanning resolution of the system by using an additional or secondary mirror that dithers the green laser pulse prior to it being transmitted, and an improvement in the steering of the receiver beam by using a controllable shutter within the receiver for the green laser pulse.
A characteristic of both those systems is that the primary mirror needs to be rotated around two axes with a quick response time. The primary mirror is relatively heavy and in older systems it has been difficult to provide a continuous rotational motion without failure or unnecessary complexity.
It is an object of the present invention to improve or at least provide the public with a useful alternative by providing a novel way of oscillating the mirror about at least one of its axes.